Astrophysicists Test Cosmological Defect Detector

Astrophysicists have built and tested the building blocks of a global detector capable of spotting topological defects in the cosmos as the Earth passes through them

March 27, 2013

One of the hot topics in cosmology these days is the possibility that the universe may be filled with topological defects left over from earlier times. The thinking is that soon after the Big Bang, the universe underwent a number of phase transitions in which the laws of physics were broken in various ways. When this happens, certain configurations of matter can end up persisting, becoming ‘frozen’ for the rest of eternity

These so-called topological defects take the form of monopoles, cosmic strings and domain walls. Nobody has observed these objects but their discovery would be a huge triumph for the theories that predict them.

Today, Szymon Pustelny at Jagiellonian University in Krakow, Poland and the University of California, Berkeley, with a number of buddies reveal their plan to spot topological defects called domain walls using a global network of sensors.

Domain walls occur at the boundary between very weak dark matter fields that became trapped in space soon after the Big Bang. These fields should interact weakly with ordinary matter but the effect is only visible when the field direction changes, a phenomenon that occurs when the Earth moves through a domain wall separating different regions of the field. When this happens, the atomic spin of every atom on the planet should wobble very slightly.

So if physicists can find a way to measure and compare these spins in different places on Earth, it ought to be possible to spot the Earth passing through a domain wall. However, that’s a big ask given that the best estimates suggest this ought to happen at the rate of about once a decade.

Pustelny and co say that recent advances in magnetic field sensors mean that exactly this kind of experiment is now possible. And they’ve built and tested a prototype to prove it.

The idea is to create a global network of magnetic field sensors that monitor the spins of atoms at various magnetically shielded locations around the world. The network must be capable of spotting and filtering out the influence of the large environmental magnetic fluctuations such as those from the solar wind. Once that is done–no easy task–it should be possible to see atomic spins around the planet shimmer in near unison as the Earth passes through a domain wall.

The prototype network consists of two sensors of this type in Berkeley, California, and in Krakow Poland. These sensors are called optical magnetometers. They work by bathing atoms in light and watching how the absorption and emission of photons changes as the atoms precess in any magnetic field.

The data from both experiments needs to be time stamped so that it can be compared for evidence of a wall crossing, an event likely to occur over a period of just 10 millisecond once every ten years.

Pustelny and co say their results prove that a global network capable of spotting domain wall crossings is feasible, “It is shown that the network consisting of sensitive optical magnetometers is capable to probe an axion-like-ﬁeld parameter space unconstrained by other experiments,” they say. They call this experiment GNOME–the Global Network of Optical Magnetometers for Exotic physics.

That’s the starting point for a new era of science. Until now, the searches for evidence of dark mater have all looked for evidence of impacts between dark matter particles and ordinary matter particles. GNOME looks for an entirely different effect: evidence of the field in which these particles must exist.

That will provide physicists with an exciting new way to look for science beyond the standard model of particle physics, a tool that is desperately needed given the increasingly predictable results from conventional particle physics experiments.